feat(stdlib): Tiled P6 — infinite/chunked maps, .world stitching, base64+zstd (#74)

- Infinite/chunked maps: <chunk x y width height> (TMX) and JSON chunks[]
  (default 16x16) decode and flatten into the dense layer array, sized to the
  chunk union; the map's 0/0 header dimensions fall back to the flattened bounds.
- .world stitching: Tiled.world / Tiled.world_count / Tiled.world_map read a
  .world (JSON, reusing Json.parse) and list its member maps at their offsets.
- base64+zstd: a self-contained pure-Ludic Zstandard decompressor
  (runtime/native/zstd.ludic, RFC 8878) — the design's "largest single item,
  explicitly last". Frame header + raw/RLE/compressed blocks; raw/RLE and
  direct-weight Huffman literals; the full FSE sequence path (predefined,
  transcribed exactly from zstd's hardcoded tables since they're not rebuildable
  from the default distributions; RLE; FSE-described) with repeat offsets and
  execution. Decodes the low-entropy GID streams a tilemap produces; a high-
  entropy FSE-compressed-Huffman-weights block fails cleanly with -1 rather than
  emitting wrong bytes (documented scope).

Proven by library/tiled_p6.ludic (12 assertions): TMX + TMJ chunk flattening,
.world offsets, and a real zstd-compressed tile layer decoding byte-exactly to
its CSV baseline. x test: 97 passed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/language/tiled/tiled-world_count.md

@@ -0,0 +1,14 @@
+---
+id: tiled-world_count
+name: Tiled.world_count
+category: tiled
+kind: namespace-method
+tokens: Tiled.world_count
+sig: Tiled.world_count(world) -> int
+tip: How many maps a world stitches.
+order: 38
+ns: Tiled
+member: world_count
+---
+
+Returns the number of member maps in a loaded <code>.world</code>.